1. Field of the Invention
The present invention relates to a method for forming an interconnect of a solid oxide fuel cell, and more particularly to a powder metallurgy method for forming an interconnect of a solid oxide fuel cell.
2. Description of Related Art
High temperature solid oxide fuel cells (SOFCs) comprise what is referred to as oxygen-ion conducting solid state metal oxide electrolyte, which can be, for example, cubic phase stabilized zirconia, with operating temperatures ranging from 700° C. to 1000° C., using as a fuel hydrogen from either natural hydrogen-rich materials such as alkanes or regenerative materials such as bioethanol. Under high temperature, hydrogen gas is produced by chemical reformation to prevent the use of noble metal catalysts. Due to the characteristic of high temperature, the SOFC has a high tolerance among the various fuels currently utilized. Dependence on the single fuel source of conventional electrical power generation can be relieved, as one advantage of the SOFC. The degree of being able to directly generate electrical power by way of the chemical electrical power provided by the SOFC is higher than that of current electrical power generation equipment using mechanical power transformation. The electrical power generating efficiency of the SOFC could reach more than 40%. Furthermore, due to the high operating temperature of an SOFC, exhaust heat energy of SOFC reactions can be recycled for cogeneration so as to increase the total efficiency of electrical power generation of the SOFC up to 80%.
FIG. 1 shows a conventional arrangement of a flat plate SOFC. The flat plate SOFC comprises an anode 10, a solid oxide electrolyte 12, a cathode 14, an interconnect 16 for holding the SOFC (also known as a bipolar plate), a sealant, etc.
In the depicted SOFC arrangement, the interconnect 16 places the cathode and the anode of two adjacent cells respectively, into electrical communication with one another. Since a single fuel cell, including a cathode, an anode and an electrolyte, can only generate a limited amount of electrical power, a plurality of single fuel cells are connected in serial to generate enough voltage for efficient utilization. Thus the material of the interconnect must have a high conductivity under both cathode and anode environments. Furthermore, the interconnect must isolate the fuel from air, and the material must be condensed and hermetic. Because the interconnect 16 is usually exposed under the environments of oxidation and reduction, the material of the interconnect 16 must have chemical stability under both atmospheres. Moreover, since the operating temperature of the SOFC is high, the material of the interconnect must have superior stability against high temperature oxidation.
The solid electrolyte layer (cf. 12), having been dimensioned in early stages of conventional SOFC development to be much thicker than that of later designs, often results in poor ion mobility and inadequate cell performance. Therefore, operation of the SOFC must be at an ultra high temperature of around 1000° C. so as to improve the cell performance. Accordingly, the early-developed interconnect (cf. 16) is made of high temperature resistant ceramic, such as LaCrO3-based ceramic. However, ceramic materials are fragile, hard to manufacture, and expensive (80% of the total production cost of the flat plate SOFC). Following such ceramic implementations came metal interconnects leading to reduction of the thickness of the electrolyte of SOFC, newly discovered electrolyte, and the decrease of the operating temperature down to 800° C.
Compared to ceramic, metal offers high electrical, thermal conductivity, easy manufacture and low cost. High temperature alloys commonly used for interconnects of SOFCs include chrome base alloy, nickel base alloy and iron base alloy (stainless steel), wherein the thermal expansion coefficient of nickel base alloy does not match other components of SOFC, and only the thermal expansion coefficient of ferrite stainless steel matches other components of the SOFC so as to be suitable for operating temperatures in a range of 600° C. to 800° C. The suitable operating temperature for chrome base alloy is near 1000° C.
Therefore, a new method of manufacturing an interconnect of chrome base alloy is needed to improve product quality and decrease production cost.